This invention relates to a cold rolled steel having high strength and excellent ductility and formability.
Cold rolled steels are used extensively in sheet applications for automotive industries. Although a low carbon cold rolled steel has excellent formability, its low strength requires thick sections for load-bearing applications. New compositions with new processes have been developed to improve the strength of low carbon cold rolled steels and to reduce the weight of vehicle. Currently, high strength cold rolled steels with a strength of 45 to 70 kgf/sq.mm have been available in automobile industries in sheets that are 0.8-1.6mm thick.
For the safety of the riders and for reducing the weight of the vehicle, still higher strength (i.e. over 70kfg/sq.mm) of a cold rolled steel is necessary for use in sections of automobiles, the bumper and the reinforced beam inside the doors. There are many kinds of cold rolled steel sheets such as solid solution hardening steel, precipitation hardening steel, recovered steel, dual phase steel and full martensite steel, which have been developed to improve strength. However, the ductility in these steels gets worse as the tensile strength increases.
In Transaction ISIJ Vol.27, (1987), Osamu Matsumura, Yasuharu Sakuma and Hiroshi Takechi describe a steel which consists of 0.4% of C, 1.5% of Si and 0.8% Mn and which is subjected to an intercritical annealing after it is hot rolled and then cold rolled. The intercritical annealing is performed by heating the steel to a temperature of ferrite-austenite phase, then cooling it to the bainite transformation temperature region followed by holding for a suitable time before air cooling. A composite structure having ferrite, the retained austenite and bainite or a little amount of martensite is obtained by this process. Because this steel contains a large amount of retained austenite, a good combination of strength and ductility is obtained through the effect of TRIP, (transformation induced plasticity). Although this steel exhibits high strength and high ductility, the weldability thereof is still unsatisfactory due to the carbon content of 0.4% by weight which is greater than an appropriate weldable carbon content.
U.S. Pat. No. 4,561,910 discloses a hot rolled steel which consists of a steel having a composition consisting of 0.03-0.15 % by weight of C, 0.6-1.8% by weight of Mn, 0.04 -0.2% by weight of P, not more than 0.10% of Al, not more than 0.008% by weight of S and unavoidable impurities. This steel is hot rolled under a condition that the heating temperature is kept at 1,10-1,250 deg C., the finishing hot rolling temperature is kept to 800-900 deg C., the cooling rate from beginning of cooling following to hot rolling to coiling is kept to 10-100 deg C./sec. The resulting hot rolled steel sheet has a microstructure consisting of ferrite and martensite dispersed therein, the area fraction of the ferrite is at least 70% and that of the martensite is at least 5% at that section of the steel sheet. The steel sheet has a yield ratio of not higher than 70%, a yield strength of at least 30 kgf/sq.mm and a tensile strength of at least 50 kgf/sq.mm.